Halaku Khan attacked Baghdad by the advice of Khojah Nasir-uddin Tusy, the great Persian astronomer and mathematician. Nasir-uddin had entered the service of the last prince of the Assassins only for the purpose of avenging himself on the Khalif, who had disparaged one of his works. When, however, he became aware of Halaku's power, he not only betrayed his new master to him, but led the Mughal conqueror also to Baghdad. After the burning of the library at Alamut (the stronghold of the Assassins, where they kept their literary treasures) and the sacking of Baghdad by Halaku Khan, the erection of the astronomical observatory at Maragha, under the direction of Nasir-uddin Tusy, was the first sign that Arab civilization and the cultivation of science had not been entirely extinguished by Tartar barbarism. The learned viziers who stood by the side of the conqueror, such as the two brothers Juvaini, were Persians, and therefore hardly belong to the history of Arab literature. But the fact that one of these two historians now wrote 'The Heart Opener,' also implies that the invasion of the barbarians had not quite put an end to literary activity. More than ten historians flourished at the beginning of this period whose names terminated with 'din,' such as Baha-uddin, Imad-uddin, Kamal-uddin, etc., and they were contemporaries of the Arab Plutarch Ibn Khallikan, already mentioned and described in the preceding period. The 'Alfiyya,' or Quintessence of Arab Grammar, was written in verse by Jamal-uddin Abu Abdallah Muhammad, known under the name of Ibn Malik. The author died in A.D. 1273-1274; but his work has lived, and it is looked upon as a good exponent of the system. The Arab text has been published, with a commentary upon it in French, by Silvestre de Sacy, A.D. 1834. During the eighth century of the Hijrah (A.D. 1301-1398), there lived three distinguished men, one famed as a geographer and traveller, and the other two as historians, viz., Ibn Batuta, Abul Feda, and Ibn Khaldun. The first-named left his native town, Tangiers, in A.D. 1324, and travelled all over the East, performing his pilgrimage to Mecca in A.D. 1332. The travels of Ibn Batuta were translated by the Rev. S. Lee, and published by the Oriental Translation Fund, as their first work, in A.D. 1829. This traveller has been noticed by Kosegarten in a Latin treatise, and his travels have been also translated into French, with the Arabic text above, by C. Defremery and R. Sanguinetti, at the expense of the French Government (1874-1879). Abul Feda Ismail Hamawi is well known as an historian, and is frequently mentioned by Gibbon as one of his authorities. He wrote an account of the regions beyond the Oxus, and also an abridgment of universal history down to his own time, and as he is supposed to be very exact, and his style elegant, his works are very much esteemed. He died A.D. 1345, having succeeded his brother Ahmad as King of Hamat in Syria, A.D. 1342.